The invention pertains to the field of variable output voltage power supplies with the capability to step up or step down the output voltage relative to the input voltage. More particularly, the invention pertains to the field of flyback power supplies with high output power capability.
In power supply design certain characteristics are very desirable. Among these characteristics are low cost, low noise on the output voltage, a variable output voltage which can be stepped up or stepped down relative to the input voltage, and the ability to have any output voltage within the designed range of output voltages for any input voltage within a designed range of input voltages which cover the world's power line standard voltages. Further, in many applications such as laser power supplies, it is very important to have low audible noise generated by the power supply. For example, in doctors, offices, lasers are frequently used to do operations on the eye. If the power supply switching regulator switches at a frequency within the typical ability of most people to hear, e.g., 16,000 Hz, and this switching frequency is imposed upon the D.C. output as ripple current, audible noise will be heard when the laser is running. Further, for laser power supplies, any noise which appears on the D.C. output will also appear in the light intensity at the output of the laser. For many applications this is highly undesirable.
Also in laser power supplies for plasma tubes used in certain types of lasers, higher levels of power output from the laser require more voltage to drive the plasma tube. Frequently, the changes in power of the laser must be made rapidly. Therefore it is desirable to have a power supply whose output voltage can be raised quickly to a voltage higher than the input line voltage and which can be varied rapidly either above or below the input line voltage.
Since there are many different applications for power supplies in the world, it is desirable to have a power supply with a modular design such that power supply modules may be ganged together in parallel or series to provide higher output powers or higher output voltages. Further, it is desirable to be able to gang the modular power supplies together in series at their inputs so that higher levels of input voltage from the power lines in certain countries may be used without the need to use a different power supply. Such a modular design allows the same module to be used in many different applications thereby allowing the annual production level for the module design to be higher. Economies of scale then allow the modules to be built more cheaply.
Finally, in some countries of the world such as the United States, there are strict standards for maximum levels of radiated emissions and for the levels of current flowing in safety ground wires for the three-wire safety ground outlets which are now required by law for many applications. Therefore, it is useful to have a modular power supply design which can meet these safety and emissions standards.